Add TextBox values to ListBox in ASP.Net on Button Click

 <asp:TextBox ID="txtNum1" runat="server" Width="100"></asp:TextBox>
        <br />
        <asp:TextBox ID="txtNum2" runat="server" Width="100"></asp:TextBox>
        <br />
        <asp:ListBox ID="lstSum" runat="server" Width="100"></asp:ListBox>
        <br />
        <asp:Button ID="btnAddResult" runat="server" Text="Add Result" OnClick="OnAddResultClick" />

   DataTable dataTable = null;

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                if (ViewState["Values"] == null)
                {
                    DataTable dataTable = new DataTable();
                    dataTable.Columns.AddRange(new DataColumn[1] { new DataColumn { ColumnName = "Sum" } });
                    ViewState["Values"] = dataTable;
                }
            }
        }

        protected void OnAddResultClick(object sender, EventArgs e)
        {
            int sum;
            sum = int.Parse(txtNum1.Text.Trim()) + int.Parse(txtNum2.Text.Trim());

            dataTable = (DataTable)ViewState["Values"];
            dataTable.Rows.Add(sum);
            ViewState["Values"] = dataTable;
            lstSum.DataSource = dataTable;
            lstSum.DataTextField = "Sum";
            lstSum.DataValueField = "Sum";
            lstSum.DataBind();
        }